Using a Punnett square, we can figure out the possible blood types of the offspring of parents even when we don't know the exact genotypes of the parents. For example, let's say the parents have type O and type A blood. We know the genotype of the type O parent--ii--but the parent with type A blood could have one of two genotypes, IAi or IAIA. That means we have to do two Punnett squares.

Now, let's say that these parents have a child with type O blood. What does that tell us about the type A parent?

Answer:
His or her genotype is IAi.

Now that you've learned a little about genetics, let's take a look at the building blocks of genes: DNA.